Boom starting to fail, repair guidance.

[dron]

I've noticed a hairline crack in the boom of my Fredy, this is about 150 degrees around. I do know it's had a repair done in the past but I'm not sure if this is exactly the same spot. The boom itself does not feel weak, with force I can expand the crack perhaps to double width, with force in the opposite direction it dissappears. There is a ballast tube which ends about 30mm fore of it. 

I've seen plenty of guidance on repairing a snapped boom but it's not snapped, yet. 

Can anybody help with the best way of repairing this? Durability is more important to me than appearance. Oh, and I'd much rather hear that I don't need to snap it first!! I can't do that in cold blood.

Many thanks in advance

mark

 

 

[wookman]

1. Cyno the crack. Hope for the best. Repair it properly if (when) it breaks again.

or 2. Cyno it and put a wrap of carbon or glass around the outside. Ugerly!

or 3. Yep you guessed it, cyno it. Sand back an inch or so either side and either end of the crack. Taper the sanding so that the fuz is paper thin where the crack was. lay in lots of thin layers of cloth and the minimum amount of epoxy to wet it out. Masking helps contain the mess. Mask the entire fuz except the bit you are repairing otherwise you will get epoxy finger marks on it, trust me you will. Cure. Sand. Paint. Good as new!

[dron]

Excellent. Thanks Wookman, I'm going to have a go at 3, and hold you to your guarantee of "good as new"  .

Does it matter what type of epoxy I use? I've only used 30 minute stuff for my general building/fixing.

[PaulR]

I'd go for option 1 (use thinnest CA to wick into all fractures and make sure its properly aligned first) and fall back to option 3 if/when needed, but if you are going for option 3 I'd suggest this resin:
https://www.easycomposites.co.uk/el2-epoxy-laminating-resin Relatively cheap and available in sensible quantities. Choice of fast or slow hardener depending on pot life needed too.

This unidirectional tape saves you chopping up pieces of cloth and I've used it to great effect on several DLG booms: https://www.easycomposites.co.uk/80g-15mm-carbon-fibre-spread-tow-ribbon

I did this one the week before last. Only two or three layers required for the strength I needed. Fibres should be in line with the boom, not wrapped around it. Compress tightly and leave to harden. Lots of good youtube videos around of this kind of repair. The raw carbon look doesn't stick out so much on the unfinished DLG boom.

This is an older one:

[wookman]

5 hours ago, dron said:

Does it matter what type of epoxy I use? I've only used 30 minute stuff for my general building/fixing.

Yes see above post

[f3fman]

Thin cyno and keep an eye on it 

Regards,

Lazy Bugger

[Phil.Taylor]

On 25/08/2023 at 08:40, wookman said:

1. Cyno the crack. Hope for the best. Repair it properly if (when) it breaks again.

or 2. Cyno it and put a wrap of carbon or glass around the outside. Ugerly!

or 3. Yep you guessed it, cyno it. Sand back an inch or so either side and either end of the crack. Taper the sanding so that the fuz is paper thin where the crack was. lay in lots of thin layers of cloth and the minimum amount of epoxy to wet it out. Masking helps contain the mess. Mask the entire fuz except the bit you are repairing otherwise you will get epoxy finger marks on it, trust me you will. Cure. Sand. Paint. Good as new!

This one - No.3

(I showed him how to do it!)

You need laminating resin - 30min wont work - Bucks composites sell smallish quantities - and carbon, and glass

https://www.bucks-composites.com/products

You need as much material - carbon/glass - across the break as the original layup material thickness. Looks like the previous repair didnt use enough glass/carbon, which is why it has cracked again.

Phil.

[wookman]

7 hours ago, Phil.Taylor said:

I showed him how to do it!

You most certainly did and a damn useful lesson it was too. 

[dron]

1 year on......
A little story, predominantly to say thanks to those who provide building advice on this forum and specifically to those who gave me that advice on my Fredy.

My initial intention was to have a go at a decent repair job but a lack of courage (I'm still a balsa/ply guy) and time resulted in the lazy option of CA. Next flight, another not too heavy landing but it resulted in a distinctly wonky fuz. It had cracked again but in a different place. It wasn't just the cracked fuz that was the problem, the flights were horrendous with skew/stall out of a loop and an elevator response that was about 1 second (and worsening) behind my action which made for a rather interesting dozen or so laps attempting to land. Into the garage it went for a winter of abandonment.

Anyway, recently I decided have a go at sorting it out. I found the elevator problem and re-positioned the servo. I'd read that the Fredy can be "flicky" and that shimming the v-tail can help so presumably I could sort that, now into the unknown. I read the repair guidance for the fuz and attacked with abrasion. It soon became apparent why the cracks had occured, there was a right bodge underneath the shiny paint, but no going back now, fixing stuff had been ordered, on with the sanding and then repair.

First photo shows the various multicoloured materials I found beneath the paint, I think that was taken after a thin layer of glassfibre/epoxy to provide a decent and uniform (ish) shape to work to. 

Next photo is where I ended after building up with successive layers of carbon tow. It felt strong enough to me.

Lessons learnt, (not to supercede any other advice out there, just key points in case anybody reads this who is in the same tentative position I was). 
i) laminating resin does get everwhere just as I was warned, masking it all off saved a right mess.
ii) wetting properly is so important. I knew I had too much with the first layer but couldn't believe how much you can squeeze out of carbon tow. An old store card dragging along the tow and squeezing resin into and through the fibres worked for me, once wetted with minimal resin left on, the tow almost seems to grab itself into position, too much and it does not. 
iii) compression - I used a strip of Bacofoil's cling film. As cling film goes this stuff is pants, it simply doesn't cling, but it does stretch and doesn't puncture as easily as some. I was able to wrap a pre-cut strip increasingly tightly with limited wrinkling. On top of that, I used my old surgical stocking to pull tighter!
iv) heat source for curing - I ended up leaving it over the back of the freezer in the garage, worked great, see 3rd photo.  

I've flown the Fredy on two days since, both times in a good breeze, the second day was with ballast and a fair bit of chucking around. I think I must have realigned the fuz properly as flight characteristics were superb, no funny skew/stall from loops. After landing numerous times to fiddle with the set up, despite some not so delicate returns to mother earth, the repairs were sound. Happy bunny.

So, a long story to say thanks, but if the forum and the contributors weren't there, that fuz was going in the bin and I'd have been a grumpy old ****. 

guys. 

[John Minchell]

I appreciate you have done your repairs now but take a look at this training video from Easy Composites as fuz booms are the same as fishing rods.  And the EC rod repair kit has all you need in it too.  And the real asset is the heat shrinkable wrapping tape. 

https://www.easycomposites.co.uk/learning/how-to-repair-a-broken-carbon-fibre-fishing-pole 

https://www.easycomposites.co.uk/fishing-pole-repair-kit

https://www.easycomposites.co.uk/composites-shrink-tape

John M

 

[Phil.Taylor]

Well done ! - good repair

The flickiness / skew/stall etc - you just need to fly it faster - get more speed on = more fun. Also - limit the elevator movement - probably +/- 4mm maximum (on the bigger F3F planes I have 5mm max) - "rudder" can be more

Phil.